Efficiency of association of scab control methods on resistance durability of apple: the case study of cultivar Ariane
Abstract: The major resistance gene Vf has been deployed in several commercial cultivars. This resulted in the emergence of virulent isolates of Venturia inaequalis in Europe. In France, isolates virulent to Vf developed since 1995, mainly in North-West region. To increase the durability of resistance of Vf cultivars in regions where virulent isolates are not yet present (or present at a low frequency), it was recommended that leaf litter be destroyed in winter and to apply fungicides at times of the highest scab risks. These recommendations, however, had not been evaluated experimentally previously. In 2004, we initiated a project to test these recommendations on cultivar ‘Ariane’, which has been deployed in France since 2002. Our objective was to evaluate the efficiency of association of scab control methods to delay the breakdown of Vf. In an experimental orchard planted with ‘Ariane’, we compared scab development in 3 untreated and 3 treated plots. In untreated plots, scab increased quickly to 98% of scabbed trees and 35% of scabbed fruits in 2008, showing the high susceptibility of ‘Ariane’ in case of breakdown of its resistance. In treated plots, destruction of leaf litter was performed each year, and 5 to 9 fungicides were sprayed each spring to cover medium and high risks of scab development following Mill’s curves. For comparison, about twice the fungicide sprayings were applied in the same region on susceptible cultivars. In 2008 on the treated plots, 4% of the trees presented a very low severity of disease, and 0.2% of scabbed fruits were observed. This study shows the efficiency to associate sanitation and reduced number of fungicide sprayings to complement Vf resistance and delay its breakdown.